Recovery furnaces which utilize black liquor for fuel are well known in the art. In general, these recovery furnaces have a boiler section which converts heat of combustion of the black liquor into steam. The boiler section is generally made up of a series of drums and heat exchange tubes through which water and/or steam is circulated under pressure. The combustion reaction in the furnace creates heat which converts the water or steam in the boiler section into high pressure steam which is then used to drive a turbine generator to produce electricity.
In the papermaking industry, spent or "black" liquor, is produced as a by-product of the kraft papermaking process. The black liquor is used to fuel a recovery furnace in the paper industry, as it is a relative high fuel value by-product which otherwise would be wasted. The inorganic components of the black liquor are recovered for re-use in the Kraft wood pulping process.
The Kraft process utilizes "white" liquor which contains chemicals for digesting wood chips to obtain pulp. The active chemicals in the white liquor are sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S). Wood chips are added to the white liquor so as to digest the lignin which holds the wood fibers together. The mixture after cooking is then separated, with the resulting pulp being sent to a paper processing facility and the residual black liquor to the recovery furnace for use as fuel and recovery of chemicals.
One of the problems which arises in the recovery furnaces which burn black liquor is the accumulation of deposits on the outer surface or "fireside" of the recovery boiler section. The evaporation and burning process of black liquor in the recovery furnace creates hydrolysis salts called "salt-cake", primarily composed of sodium sulfate (Na.sub.2 SO.sub.4) and sodium carbonate (Na.sub.2 CO.sub.3). These salt residues, generally consisting of about 70% Na.sub.2 SO.sub.4 and 30% Na.sub.2 CO.sub.3 are deposited on the heat exchange tubes, and thus foul the upper surfaces of the boiler section, thereby insulating the heat exchange tubes from the heated flue gases generated by the recovery furnace and, in extreme cases, obstructing the upper boiler section gas passages.
The salt-cake deposits which form on the upper surfaces of the boiler as a result of the evaporation and burning of black liquor present a significant problem in maintaining the boiler's thermal efficiency. In order to remove the salt-cake deposits, "blowers" have been developed to remove the deposit from the upper surfaces of boilers and recovery furnaces. See G.A. Smook, Handbook for Paper and Pulp Technologists, pp. 134-135. Soot blowers utilize high-pressure steam to mechanically remove the deposits from the tubes. It is necessary to regularly engage mechanical soot blowers in a recovery furnace to remove the deposits from the upper surfaces of the boiler section.
Often, the gas temperature in a recovery furnace is sufficiently high to cause the hydrolysis salts and ash particles in suspension to become sticky and tacky. When this occurs, the deposit fouls the superheater structure of the boiler section, the transport tubes and other upper boiler sections. When a deposit is sticky and tacky, which is typical in overloaded situations, it cannot be controlled with mechanical soot blowers. Additionally, when the deposits become thick enough, they can block the passage of combustion gases, thereby preventing the boiler section from functioning properly. The deposits then become hard and extremely difficult to remove with a mechanical soot blower.
The soot blowers furthermore use steam provided by the boiler section to remove the deposits. A significant portion of the steam which could otherwise be used to drive the turbines to produce electricity must be diverted for use in the soot blower to remove the deposits from the heat exchange tubes. This has a distinct disadvantage in that a substantial portion, sometimes up to 5% of the energy output of the recovery boiler, is used for the operation of the soot blowers.
Additionally, when soot blowers are ineffective to completely remove salt-cake deposits from the heat exchange tubes, the recovery furnace must be shut down until the cleaning operation is completed. Thus, valuable time is lost in this deposit removal method.
There is a recognized, long-felt need in the art for improved methods and apparatus to remove deposits , from the upper surfaces of a boiler since conventional mechanical soot blowers cannot efficiently accomplish this task.
Various methods and devices have been suggested to remove deposits from heat exchange tubes in boilers. An example of a class of these devices can be found in U.S. Pat. No. 4,018,267, Tomasicchio. Tomasicchio discloses methods and apparatus which shake or strike the deposit covered surfaces in a boiler in order to try and dislodge solid deposits on the tubular arrays therein. Similar to the devices disclosed in Tomasicchio are the devices disclosed in U.S. Pat. No. 4,497,282, Neundorfer. The devices disclosed in Neundorfer apply high frequency shock energy to tubes in a steam generator in order to "de-slag" the tubes.
The devices disclosed in Neundorfer and Tomasicchio have been found to be unsatisfactory since the devices disclosed in these references require application of high-energy shock waves which can damage and dislodge the heat exchange tubes in the boiler section and other equipment located within the recovery furnace. Furthermore, the device disclosed in Neundorfer and Tomasicchio require substantial additional apparatus within the recovery furnace itself in order to accomplish the task of cleaning the deposits from the tubes. This requires substantial capital investment in additional equipment and considerably more time and effort in maintenance.
Examples of standard mechanical soot blowers can be found in U.S. Pat. No. 4,421,067, Krowech. The devices disclosed in Krowech utilize a rotary soot blower tube coupled to a valve-controlled pneumatic actuator. This device is then fixed to the vessel which it is intended to de-slag. The valve-controlled pneumatic actuators disclosed in Krowech move a soot blower tube back and forth against the vessel as the soot blower ejects steam to clean the vessel walls. This motion is intended to loosen the deposits along the vessel walls so that the standard mechanical soot blowing action can more easily remove the deposits.
The devices disclosed in Krowech fail to satisfy the requirement for a device to remove heavy deposits from heat exchangers and upper boiler surfaces since they generally can only loosen the loosely adhered deposits. The mechanical actuators disclosed in Krowech are also potentially damaging to the heat exchange tubes and vessel walls.
It is also known in the soot blower art to utilize water jets to provide slag removal. However, the use of a water jet is generally impractical for deposit removal since it is difficult to control and limit the thermal shock of the water jet against the tubes to prevent premature failure of the tubes. See, e.g., U.S. Pat. No. 4,422,882, Nelson et al., at column 1, lines 14-25. The devices disclosed in Nelson et al. require delivering liquid from a high pressure source against soot deposits on boiler section tubes in a pulsed manner. Additionally, it is impractical to use water jets in a recovery furnace used in the paper industry due to the high risk of explosion if water contacts molten slag in the recovery furnaces. Thus, the devices disclosed in Nelson et al. run the high risk of rupturing the tubes as the high pressure liquid impinges on their surfaces. Furthermore, depending upon the tenacity of the soot deposits lodged to the tube, the devices disclosed in Nelson et al. will not efficiently remove all of the deposit. Thus, the devices disclosed in Nelson et al. do not satisfy the requirements for safe and efficient removal of deposits from the upper surfaces of a boiler.
Lasers have been used in the past to remove unwanted materials from surfaces. An example of such an application can be found in U.S. Pat. No. 4,368,084, Langen et al. The devices disclosed in Langen et al. comprise laser beams which are focused on metallic objects having a coating of rust. The lasers pulse coherent light energy on the rust which then evaporates.
Other uses of lasers to clean surfaces are disclosed in U.S. Pat. No. 3,503,804, Schneider et al. The devices disclosed in Schneider et al. teach the use of laser beams which agitate a liquid jet to produce sonic cleaning of the surface. These devices, like those disclosed in Nelson et al., involved the use of water, which is intolerable in recovery furnaces which contain molten slag, such as when burning black liquor.
Thus, the devices disclosed in Schneider et al. and Langen et al. do not satisfy the requirements for devices which can safely, efficiently, and consistently remove deposits from heat exchange tubes found in the high-temperature boiler.
It has been known to use lasers to remove slag deposits which are generated in the melting chamber of a lower furnace section. See German Patent 3243808. The German patent discloses use of a laser to ensure that the discharge opening of a melting chamber in a furnace remains open. Melting chambers are found in the lower parts of a furnace used in coal power plants and are used to remove slag buildup in the lower parts of the furnace. The devices taught in the German patent do not provide a satisfactory solution for a deposit removal device to efficiently and economically dispose of hardened deposits in the upper section of boilers.
There has thus been a long-felt need in the art for devices and methods which substantially remove deposits from surfaces found in the upper boiler section of a boiler found in recovery furnaces used in the papermaking industry.